Magnetic drive



*May 8, 1956 H. J. FINDLEY 2,744,596

' MAGNETIC DRIVE May 8, 1956 H. J. FINDLEY 2,744,596

MAGNETIC DRIVE Original Fled June 4, 1949 y 2 Sheets-Sheet 2 I N V ENTOR. /90 ward f/'Ir/ay M l V-.14:4

United States Patent O MAGNETrc DRIVE Howard I. Findley, Cleveland,Ohio, assigner, by direct and mesne assignments, to Eaton ManufacturingCompany, Cleveland, (Ehio, a corporation of @hie 4 Claims. (Cl.192-21.5)

This invention relates generally to magnetic drives, and moreparticularly to magnetic clutches for controllably coupling two memberstogether and is particularly adapted, among other uses, as a magneticclutch for coupling a prime mover to a rotatable load whereby therotation of the load may be controlled. This application is a divisionof applicants copending application Serial No. 97,269, tiled June 4,1949, now Patent No. 2,519,449, granted August 22, 1950, as acontinuation-in-part of applicants copending but now abandonedapplication, Serial No. 38,543, filed July 13, 1948.

Objects of this invention are to provide such a magnetic clutch havingimproved operating characteristics; to provide such improvedcharacteristics by the addition of a bonding material comprising aquantity of nely divided magnetic material and a quantity of finelydivided solid lubricant; to provide such a high torque clutch which hasa low no-load drag; to provide such structures embodying improved meanswhich act upon the bonding material under unexcited conditions of theunit to maintain bonding material in the air gap regions, in readinessto establish the bond; and to generally improve magnetic drives of thecharacter described.

Referring to the drawings:

Fig. 1 is a view in axial section of an embodiment of the invention;

Fig. 2 is a fragmentary View illustrating a detail of Fig. l;

Fig. 3 is a fragmentary view illustrating a modied spring structurewhich may be used in the structure of Fig. 1; and,

Fig. 4 is a view in axial section of another form of plate clutchembodying the invention.

In accordance with the broader aspects of the present invention, it maybe applied to a wide variety of magnetic clutch constructions, whichconstructions may be generically classified as including a plurality ofrelatively rotatable members formed at least in part of magneticmaterial and so arranged as to receive between adjacent faces thereof aquantity of the hereinafter described bonding material. When energized,the bonding material appears to provide a magnetically establishedelectromechanical bond between the two members which, within certaintorque limits, causes the members to move synchronously and, beyondthese torque limits, permits a slipping movement between the driving andthe driven ones of the members.

Within the purview ofthe invention, the members themselves may bevariously constructed, the resultant torque may be due entirely or inpart to the aforesaid bond, the excitation may be variously applied, andmay be either intermittent or continuous, and the members mayinterchangeably serve as driven or driving members. To illustrate thewide adaptability of the invention, and the wide variety of structureswhich may embody it, a number of different clutch structures aredisclosed herein, in an illustrative and not in a limiting sense.

The magnetic clutches of Figs. 1 through 4 are of the 2,744,596 iatentedMay 8, 1956 plate type, in the inter-plate region of which has beenplaced a quantity of ne magnetic particles of the order of 8 microns indiameter, and a quantity of solid dry powdered lubricant such asgraphite, mica, soapstone, potstone, steatite, French chalk, lampblack,and so forth, or mixture thereof. The presence of the magnetic particlesin the air gap tremendously increases the torque which may betransmitted by a unit of given size. The use of the dry lubricantpermits the operation of the unit under slip conditions without undueinternal losses and particularly at non-excited or no-load conditions.The addition of this powdered mixture permits synchronous rotation ofdriving and driven members in which condition there is no eddycurrentdriving efect between the two members and the sole driving effectappears to be that produced as a result of the presence of the magneticmaterial in the air gap. It further permits operating under slipconditions in which the dry lubricant prevents undue abrasion andwearing of the unit, and no-load or non-excited operation with full slipin which the dry lubricant and magnetic particles offer little if anyresistance to relative rotation of the driving and driven members.

The exact theory of the action of these materials, defined below asbonding materials, is not known with absolute certainty, but the resultsare definite and positive. It is believed that the presence of thispowdered material in the coupling acts, under energized conditions, toprovide pillars or columns lying in the flux paths and which are formedand held together by the ux flowing in such paths so that the drivingmember is connected mechanically and frictionally to the driven memberby means of such columns. Under deenergized conditions, and except asnoted below, the components of the bonding material have completefreedom of movement and so do not cause a drag between the tworelatively movable parts of the coupling.

Throughout this specification and claims the term powdered bondingmaterial is used as comprising several different materials, one havingmagnetic characteristics such as iron and another having lubricating butsubstantially non-cohesive characteristics such as the solid lubricantsmentioned above, or mixtures of the lubricants. In the presentlypreferred embodiments of the invention, a preferred magnetic material isthat known commercially as Carbonyl E and a preferred solid lubricant ispowdered graphite. As aforesaid, Carbonyl E has a grain size of about 8microns, and the graphite may be of the order of 200 mesh.

Different operating conditions determine the most preferable ratiobetween the weight of magnetic particles and the particles of solidlubricant. For example, excellent results have been obtained withstructures of the type disclosed in Figs. 1 through 4, using a 50-50mixture, as measured by volume, of Carbonyl E and graphite. The mesh ofthe lubricant may vary between wide limits but experimental results withthe Various structures disclosed herein indicate that the finer thepowder the better the obtained results. Mixtures containing a higherpercentage of graphite than that given above have been used with goodresults. Similarly, in all cases, various factors determine the optimumquantity of powdered mixture in relation to the size of the magneticdrive and particularly to the volume of the air gap. Generally speaking,it is preferred to use a volume of powdered mixture which issubstantially in excess of the volume of the air gap, but which is alsoa minor fraction, twenty-five per cent or less, of the free volume ofthe unit.

Continuing with the general description of the invention, the formedcolumns which produce the bond are believed to be composed almostentirely of themagnetic material and are believed to bear against thedriving and driven surfaces of the coupling. The bearing of the columnsso formed against these surfaces apparently sets up a frictionalengagement therebetween which opposes relative movement between thesecolumn ends and the surfaces with which they are engaged.

These frictional forces, at least within limits, will be proportional tothe strength of the magnetic eld transmitted through the columns. Duringsynchronous operation of the structure of Fig. l these columns are theonly driving force between the driving and driven members. Under slipconditions of operation, however, it appears that the pillars no longerengage the cooperating surfaces with sufficient force to causesimultaneous rotation between the driving and driven members. Under thisoperating condition, slipping is believed to occur between the ends ofthe magnetic columns and the surfaces of the members.

Experiments with the units now being described show that when a powdereddry lubricant is used, as disclosed, and when the units are deenergizedthe magnetic material columns will immediately collapse and disintegrateto their constituent particles. Except as described below, theseparticles, and the lubricant particles, are somewhat dustlike incharacter and freely oat within the units, more or less as free floatingparticles which do not tend to adhere together in larger masses orgroups and which show no appreciable tendency to form a gummy substanceimposing an undesired drag between the rotor and drum. Under suchconditions there is no substantial drag imposed by one member on theother member irrespective of which member is driven.

It may thus be seen that the presence of this powdered materialincreases the torque which may be transmitted through a magnetic unitunder driving conditions; that under no-load or idling conditions, itdoes not impose an undesirable drag; and that the units operateeiciently and over long periods of time under severe loading conditions.Even under heavy duty loadings, the breakdown temperatures of theextremely stable constituents of the bonding material are neverapproached.

Moreover, because of the freedom from frictional drag under unexcitedconditions, the internal heat which is developed in the unit ismaterially reduced. Thus the overall effect of the powdered mixture isto permit of a much smaller unit for any given load and the eliminationof special provisions for cooling.

The herein disclosed embodiments of the invention illustrate forms ofthe improvements generically claimed in said Patent No. 2,519,449wherein the principal clutch elements are in axially opposed relation(more specifically, of the plate type), and in which special provisionis made to insure that an adequate amount of bonding material is at alltimes maintained in the space between the opposed clutch surfaces. lnthese embodiments, this last named feature is accomplished in two ways,which are preferably employed together but which, in the broader aspectsof the invention, may individually be relied upon to maintain thebonding material in the said space.

The first herein illustrated expedient for controlling the positioningof the bonding material comprises generally the provision of circulatingpassages of such form and arrangement that the bonding material iscaused to circulate along a path which includes the space between theopposed clutch surfaces.

The second herein illustrated expedient for controlling the positioningof the bonding material comprises generically an arrangement whereinbonding material is caused to adhere to or become a part of the opposedbonding surfaces of one or more of the relatively movable clutchelements. Specifically, as herein illustrated, this adherence iseffected magnetically by forming one or more of the opposed bondingsurfaces of material which has a substantial amount of magneticretentivity in consequence of which even under unexcited conditions ofthe unit, a

4 substantial amount of the bonding material is positively held on thebonding surface or surfaces.

The remaining embodiments also illustrate the adaptability of thepresent invention to structures employing compound air gaps and air gapswhich are increased or decreased, depending upon the unexcited orexcited condition of the unit.

Referring now to Figure l, the illustrated unit is a plate clutchdesigned specifically for use as the main clutch for an automotivevehicle, and is consequently adapted for connection between thecrankshaft and the transmission of such a vehicle. in Figure l, theclutch comprises a pair of axially spaced and relatively movable drivingshoes 152 and 154 which receive between them a driven plate 156. Shoe152 also constitutes a flywheel, and is shown as carrying a conventionalring gear 158 for cooperation with the usual cranking mechanism (notshown). The outer face of shoe 152 is centrally recessed to accommodatethe conventional associated crankshaft flange 160, and these elementsare secured together by a plurality of circumferentially distributedstuds 162.

Shoe 152 constitutes one side of the enclosing shell for the unit. Theother part of the shell is provided by a cup-shaped member 164 which tsover a shoulder provided on the member 152, and is secured in placethereon by a plurality of circumferentially distributed studs 166. Shellmember 164 is provided with a cylindrical neel; 168, which carries ashaft seal and also, as described below, serves as a slip ring.

The inner shoe 154 is ringlike in structure and has a central aperture172. Shoe 154 is connected to shell member 164, for rotation therewith,but for axial movement with respect thereto, by a plurality ofcircumferentially distributed pins 174. Pins 174 are fixed to member164, and their reduced end portions have sliding lits within aperturesprovided therefor in shoe 154.

Under unexcited conditions, the illustrated normal spacing is maintainedbetween the shoes 152 and 154 by spring means interposed therebetweenand herein illustrated as comprising a plurality of separatehairpinlilre elements 176 which are received in annular grooves providedtherefor in the faces of the elements 152 and 154. Springs 176 areconventional, and their general nature is indicated in Figure 2. Underexcited conditions, shoe 154 is drawn towards shoe 152. Genericallyspeaking, this axial movement of shoe 154 may be permitted to continueand bring the three clutch elements 152, 156 and 154 into directface-to-face engagement with each other through the interposed layer ofbonding material, proper limits to the engaging pressures beingestablished by properly proportioning the magnetizing eld strengthrelative to the related opposing spring elements. in the preferredpractice of the invention, however, a limit is imposed to the axialmovement of shoe 154, which maintains it slightly spaced from the plate156. As shown, these limiting means comprise a series ofcircumferentially distributed studs 180, which are threaded into tappedopenings provided therefor in shoe 154. Studs 180 pass freely throughopenings provided therefor in the shell member 164, vthese aperturesbeing sealed by conventional seals 182. Adjustable nuts 184 carried bythe studs 180 serve to adjustably limit the axial movement of shoe 154towards plate 156 and shoe 152. 1f desired, and as shown, springs 186may be provided on studs 18D. These springs, of course, oppose theaction of the hairpin springs 176 but are ineffective to prevent thelutter from establishing the normal unexcited shoe spacing shown in thedrawing.

The clutch plate 156 is carried by a hub 190, for rotation therewith,but for limited axial moventeif; with tcspect thereto. Moreparticularly, plate 156 is provided with a series of circumferentiallydistributed openings, which slidably receive the enlarged body portionsof a corresponding series of studs 192, which are threaded into the angeof the hub 190. Springs 194 continuously urge vextending portions 246.

the plate into direct engagement with the face of the hub 199, andnormally maintain such engagement. When the unit 'is excited, plate 156is magnetically drawn towards' the shoe 152 and as shown is caused todirectly engage the face thereof, through the interposed layer ofbonding material, with a force determined by the ratio between themagnetic effect and the opposing springs 194. When such engagement ispermitted, the face pressures are, in accordance with the invention,maintained low enough so as to prevent a breakdown of the lubricatingfilm afforded by the lubricating component of the bonding material. Asshown in Figure l, simple springs 194 are employed. lf desired, acompound spring arrangement, as shown in Figure 3, can be employed, inwhich view studs 192s are provided with spring cages 200 Which receivesomewhat heavier springs 202. Cages 260 maintain springs 202 ineffectiveuntil just prior to engagement between plate 156 and shoe 152. lt willbe appreciated that by properly proportioning springs 202, they mayserve as stops, to interrupt the movement of plate 156 just prior toengagement with shoe 152, in which event, under excited conditions,small air gaps are maintained between shoe 152 and plate 156, andbetween plate 156 and shoe 154.

Hub 190 is keyed to the inner reduced end portion of the shaft 204which, in line with conventional practice, receives bearing support fromits associated transmission. Additionally, a pilot bearing 206 ispreferably interposed between the inner end of-shaft 264 and shoe 152.This latter bearing may be and preferably is provided with a usual shaftseal 2&8.

Shoe 152 is provided with an annular recess 210 which receives theexciting coil 212.l One terminal 214 of coil 212 is electricallyconnected,as by a stud 216, to the shoe 152. Shoe 152 in turn isconnected through casing member 164 and its neck 168 to a groundingbrush 218, which is directly electrically connected to ground throughbracket 220 and the clutch housing 222.

vThe other coil lead 224 is brought out to a terminal 226, which isconnected through a wire 228 to the slip ring 230. Slip ring 230 iscarried by neck 168, but is insulated therefrom by insulating elements232. Power is brought to ring 230 through brush 234, which may beconnected in any suitable manner to the ungrounded terminal of thebattery (not shown).

As described in said Patent No. 2,519,449, different operatingconditions determine the optimum proportionings of the magnetic andlubricating components of the powdered bonding material. Generallyspeaking, the previously specified prcportionings of the components ofthe bonding material is preferred. Similarly, dilerent operatingconditions determine the optimum quantities of bonding material inrelation to the free volume of the air gap space, or spaces. Generallyspeaking, the previously indicated volume relations are preferred;namely, a quantity of bonding material which exceeds the volume of theair gap, but which is less than the free volume of the enclosingchamber.

Coming now to the arrangements for insuring the presence in the air gapregion of an adequate quantity of the bonding. material, plate 156 isprovided with a series of circumferentially distributed openings 245i,and shoe 154 is provided with a corresponding series of angularlyextending openings 222. Preferably, also, the shoe 152 is provided witha circumferentially distributed series of conducting passages defined byinterconnecting passage portions 244, 246 and 22S. rhe passages may beformed, as will be understood, by directly drilling the passage portions244 and 248 so as to communicate with the radially Portions 246 areclosed by plates 249.

As before, rotation of the driving shoes 152 and 154 cause the bondingmaterial to circulate lin a cloudlike manner. The angularly directedpassages 242 and 248 act as pumps or blowers to draw the bondingmaterial thereinto and cause it to circulate in the air gap spaces ateither side of the plate 156. As before, also, this continuouscirculation enables the dry-lubricant component of the bonding materialto build up a thin but lubricating lm on the surface of the clutchelements, including the opposed clutching surfaces thereof, therebyadequately protecting these surfaces from any abrasive effect of thevcirculating magnetic particles. These lubricating surfaces also preventabrasion of the opposed clutch surfaces when the unit is excited.

As aforesaid, a further and important feature of the invention residesin so arranging the structure thatsubstantial amounts of the bon-dingmaterial and particularly the magnetic component thereof are positivelycaused to remain on one or more of the opposed clutching surfaces. Inthe illustrated embodiment of the invention, this is accomplished byforming the shoes 152 and 154 and, if desired, the plate 156 of magneticmaterial having a pronounced amount of magnetic retentivity. As anexample, both shoes and the plate 156 may be formed of cast iron orsteel, the metallurgy of which provides the proper degree ofretentivity. As shown, the circulating passages in the shoes are largeenough to permit a free movement of the bonding material therethrough,even in view of the magnetic retentivity of these elements, in whichevent, as disclosed, the entire bodies of shoes 152 and 154 can beformed of the same material. lf desired, of course, the bodies of theseshoes may be formed of soft iron and they may be provided with opposedfaces of the material of higher retentivity. in either arrangement, evenunder excited conditions, substantial amounts of the magnetic cornponentof the bonding material are magnetically adhered to the opposed clutchfaces, and are thus continually in readiness to establish the magneticbond, as soon as the winding 212 is excited. As previously stated, inthe broader aspects of the invention, either the circulating passagearrangement or the retentivity arrangement may be individually reliedupon to accomplish the purpose of maintaining a suiiicient amount of thebonding material in the gap at all times. Preferably and as illustrated,however, the two expedients are used together.

summarizing the operation, under excited conditions of the winding 212,the yplate 156 remains stationary and shoes 152 and 154 are driven.Under these conditions, the bonding material circulates in cloudlikeform, portions thereof being caused by the circulating passages to con-.tinually circulate through the air gaps. Other portions of the bondingmaterial are caused by the retentivity of the elements to adhere to thesurfaces of the shoes or the plate, or both.

When the Winding 212 is excited, the magnetic attraction between theshoes 152 and 154, and, in case plate 156 is formed of magneticmaterial, between shoe 152 and this plate, causes these elements to movetoward each other. In the absence of restraint, direct contact isestablished therebetween, except to the extent that they are maintainedspaced by the interposed 'lm of bonding material. As illustrated,however, shoe 154 is continually spaced from plate 156, and plate 156engages shoe 152.

In addition to drawing the clutch elements toward each other, asaforesaid, the excitation of winding 212 also causes the magneticcomponents of the bonding material which are interposed therebetween, toestablish the previously identifie-d bond between these elements.

The remaining embodiment sho-wn in Figure 4 corresponds to theembodiment of Figure l, except in the respects noted below. In Figure 4,the entire unit is carried by a shaft 250, which may be either the inputshaft or the output shaft, but is usually the output shaft. Shoes 252and 254 are rotatably carried by shaft 25@ through ball bearing units256 and 258. As before, shoe 254 constitutes one side of the enclosinghousing of the unit v'and carries a cooperating shell member 260. Shoe252 is carried by shell 269 for rotation therewith, but for axialmovement with respect thereto, by pins 262. Springs 264 continuouslyurge the shoes apart and normally maintain the illustrated spacingtherebetween.

The clutch plate 266 is carried by a spool-shaped hub 268, which in turnis keyed to the shaft 250. Hub 268 is provided with a series ofcircumfercntially spaced pins 270, which slidably pass through aperturesprovided therefor in plate 266, and which consequently permit an axialmovement of plate 266 with respect to the shoes. Stops 272, carried bythe pins 270, normally hold plate 266 out of contact with shoe 252, andsprings 274 yieldingly urge the plate against the stops 272.

Shoe 254 carries the pulley 276 which may serve either as an input or anoutput member but, as stated above, normally serves as the input memberof the unit. Additionally, shoe 254 has securely fitted thereto a hub278 which serves as a slip ring for cooperation with the grounding brush28%, and which also carries through interposed insulation 282 thecompanion slip ring 284. Slip ring 284 is slidably engaged by brus 286which may be connected to the ungrounded side of the source of power. Asbefore, slip ring 284 is connected through lead 288 to one terminal ofthe exciting winding 290. The other terminal of winding 290 is directlyconnected to the shoe 254, and thence through hub 27S and brush 230 toground.

in operation, the unit of Figure 4 performs much as does the unit ofFigure l, with the exception that in this instance the magneticretentivity of shoes 252 and 254 is entirely relied upon to maintainadequate amounts of the bonding material in the air gap. Also, in thearrangement of Figure 4, the shoes and plate directly engage each otherexcept for the interposed layer of bonding material, it beingappreciated that the strength of the magnetic field is so related to theopposing forces of the springs that the contact pressures between theopposed clutch surfaces are below those which would cause any breakdownof the lubricating film afforded by the lubricating component of thebonding material. Preferably, the proportionings of the components ofthe bonding material and the amounts thereof in relation to the freevolume of the unit and the volume of the air gap spaces, corresponds tothe proportionings and volumes discussed above in connection with Figurel.

Although only several specific embodiments of the invention have beendisclosed in detail, it will be appreciated that the improvementsthereof may be embodied in other and widely differing specificstructures. The disclosure herein, accordingly, of specific embodimentsof the invention is to be taken in an illustrative and not in a limitingsense.

I claim:

l. A coupling device including at least a pair of coupling membersmounted for rotation relative to each other and having radially andcircumferentially extending opposed face portions which are normallyspaced apart from each other along the axis of said relative rotation soas to define an air gap therebetween, said face portions beingrelatively movable along said axis to thereby vary said air gap, meansadapting one of said members to act as a driving member and adapting theother of said members to apply a driving force to a load, means forestablishing a magnetic field across said air gap and thereby drawingsaid face portions toward each other and at least reducing said air gap,and bonding material comprising finely divided magnetic particles andsolid lubricant in said field and also responsive to said field forestablishing a lubricated load transmitting bond between said faceportions, whereby rotation of said one member at least tends to causerotation of said other member, said coupling device being furthercharacterized in that at least one of said members is continuouslyefiective during both the presence and the absence of the action of saidfield establishing means and during both the presence and the absence ofrelative rotation between said members to maintain enough of saidbonding material between said face portions to establish said bondtherebetween immediately upon establishment of said field.

2. A coupling device including at least a pair of coupling membersmounted for rotation relative to each other and having radially andcircumferentially extending opposed face portions which are normallyspaced apart from each other along the axis of said relative rotation soas to define an air gap therebetween and are relatively movable alongsaid axis to thereby vary said air gap, means adapting one of saidmembers to act as a driving member and adapting the other of saidmembers to apply a driving force to a load, actuating means for movingsaid members towards each other and establishing a lubricated loadtransmitting bond between said face portions, said actuating meansincluding means for establishing a magnetic field across said air gap,bonding material comprising finely divided magnetic particles and solidlubricant in said field and responsive thereto for establishing saidlubricated load transmitting bond between said face portions, and saidactuating means being effective to draw said face portions intoface-to-face engagement with each other except to the extent that thesame are separated by the bonding material interposed therebetween, saidcoupling device being further characterized in that at least one of saidmembers is continuously effective during both the presence and theabsence of the action of said field establishing means and during boththe presence and the absence of relative rotation between said membersto maintain enough of said bonding material between said face portionsto establish said bond therebetween immediately upon establishment ofsaid held.

3. A coupling device including at least a pair of coupling membersmounted for rotation relative to each other and having radially andcircumferentially extending opposed face portions which are normallyspaced apart from each other along the axis of said relative rotationand are relatively movable along said axis to thereby vary said spacing,means adapting one of said members to act as a driving member andadapting the other of said members to apply a driving force to a load,actuating means for moving said members towards each other andestablishing a lubricated load transmitting bond between said faceportions, said actuating means including means for establishing amagnetic field across said space between said face portions, stop meansacting between said coupling members to prevent face-to-face engagementbetween said face portions, and thereby maintain an air gaptherebetween, and bonding material comprising finely divided magneticparticles and solid lubricant in said field and also responsive to saidfield for bridging said air gap and establishing said lubriacted loadtransmitting bond between said face portions whereby rotation of saidone member at least tends to cause rotation of said other member, saidcoupling device being further characterized in that at least one of saidmembers is continuously effective during both the presence and theabsence of the action of said field establishing means and during boththe presence and the absence of relative rotation between said membersto maintain enough of said bonding material between said face portionsto establish said bond therebetween immediately upon establishment ofsaid field.

4. A coupling device including at least a pair of coupling membersmounted for rotation relative to each other and having radially andcircumferentially extending opposed face portions which are normallyspaced apart from each other along the axis of said relative rotation soas to define an air gap therebetween, said face portions beingrelatively movable along said axis to thereby vary said air gap, meansadapting one of said members to act as a driving member and adapting theother of said members to apply a driving force to a load, means forestablishing a magnetic field across said air gap and thereby drawingsaid face portions toward each other and at least reducing said air gap,and bonding material comprising finely divided magnetic particles andsolid lubricant in said field and also responsive to said eld forestablishing a lubricated load transmitting bond between said faceportions, whereby rotation of said one member at least tends to causerotation of said other member, said coupling device being furthercharacterized in that at least one of said members has sulicientmagnetic retentivity so that it is continuously effective during boththe presence and the absence of the action of said field establishingmeans and during both the presence and the absence 10 of relativerotation between said members to maintain enough of said bondingmaterial between said face portions to establish said bond therebetweenimmediately upon establishment of said eld.

References Cited inthe tile of this patent UNITED STATES PATENTS 811,654Murphy Feb. 6, 1906 10 Payne Mar. 22, Findley Aug. 22, Winther Oct. 10,Prince Feb. 13, Rabinow Nov. 20, Winther Jan. 1, Winther et al. June 17,Winther July 29,

FOREIGN PATENTS Belgium Nov. 13,

OTHER REFERENCES Technical Report 1213, National Bureau of Standards, 15Washington, D. C.

